Electric discharge tube



- May 2, 1944. F. COETERIER' ELECTRIC DISCHARGE TUBE Filed Feb 12, 1942 :NVENTOR Freder/k Cogferler.

Patented May 2, 1944 'UNlTED STATS ELEG'ERIC DISCHARGE TUBE Frederik Coeterier, Eindhoven, Netherlands; vested in the Alien Property Custodian Appiication February 12, 1942, Serial No. 430,602 In the Netherlands July 3, 1940 Claims.

For various purposes high-vacuum electron discharge tubes are required in which narrow, sharply limited electron beams are generated. Such tubes are useful in apparatus for oscillography and for television, and in modern transmitting tubes for generating ultra-short waves use is made of narrow electron beams. More particularly in the latter case it is essential that a sharply limited beam having the maximum possible current density should be generated, from which strongly concentrated space-charges can be formed by means of velocity modulation in order to bring about strong current impulses in an oscillatory circuit.

In cathode-ray tubes for television and oscillography attempts have been made to concentrate the electrons emitted by the cathode immediately after they pass out of the emissive surface so that no electrons get lost. It hasbeen suggested that the cathode be surrounded by a coil which is traversed by a current, so that the dispersion of the electrons will be prevented by the magnetic field thus excited. Rather poor results have been obtained in this way, since the magnetic field of such a coil is generally too homogeneous to bring about an eifective concentration of the cathode rays.

The principal object of the invention is to obtain a strong concentration of the electrons in the form of an electron beam with great current density. According to the invention, for this purpose the cathode is surrounded by a ferro magnetic body which is magnetically polarised in the direction of the electron beam. At the end of this body the magnetic field is a non-homogeneous fringe field which forces the electrons to move to the axis of the system.

The magnetically polarised body may be a permanent hollow magnet, or it may be a ferro magnetic body which is magnetised by a magnet coil or similar means.

In order to avoid as much as possible dispersion of the electrons the electrostatic lines of force, which also determine the paths of the electrons, are caused to coincide as much as possible with the lines of force of the concentrating magnetic field.

For this reason it is desirable that the emissive surface of the cathode should be arranged near the mouth of the polarised body, 1. e. in the region wherein the axial component of the magnetic field reverses its direction, while it is further desirable to use a concave emissive surface so that the electrostatic field converges sharply.

An advantageous shape of the electrostatic lines of force may be promoted by giving the magnetic body a potential which is negative relatively to the. cathode so that this body also serves as a Wehnelt cylinder.

The invention will be more clearly understood by reference to the accompanying drawing showing, by way of example, a sketch of an electrode system for generating a directional electron beam for a discharge tube according to the invention.

The system is constitutedby. a cathode l, preferably an indirectly heated thermionic cathode, a grid shaped control electrode 2 and a first anode ii. In accordance with my invention, the cathode is surrounded by a cylinder or tubular body 4 of term magnetic material whose edge 5 exhibits the properties of a magnetic pole. The body l may be a permanent magnet or it may be surrounded by a coil, not shown, which may if desired be outside the wall 6 of the tube, so that it can be traversed by an electric current to magnetise the cylinder 4.

A potential difierence is preferably set up between the cathode I and the body 4, which renders the latter negative relatively to the cathode and promotes the convergence of the electric lines of force which pass out of the emissive surface of the cathode and are even at their origin directed along the longitudinal axis of the system due to the concave shape of the said cathode surface.

Assuming that the edge 5 of the body l is a north pole, the magnetic lines of force within the body t run from the edge 5 backwards, i. e., in the figure from the right to the left. In front of the cathode, i. e., in the figure to the right of the cathode I, they run from the left to the right. Consequently, the magnetic field at the mouth of the body 4, in front of the cathode, is a fringing field and very non-homogeneous in contradistinction to the homogeneous field of the conventional magnet coil which has in the past been suggested for a similar purpose.

Due to the concave shape of the emissive surface of the cathode and the arrangement of the grid 2 and anode 3 relatively to each other the electrons in motion do not acquire a strong component normal to the magnetic lines of force so that at the point where the magnetic field in the region traversed by the electrons is most convergent and has the greatest field-intensity the electron paths can deviate very little from the magnetic lines of force.

As a result a large cathode surface can be used of, say, square millimeters and a comq 'quired. 1

I claim: 1

paratively great current can be obtained, for example 0.5 to 1 ampere with very small loss through stray electrons.

It is advisablethat the beam concentrated by the electrode system above described should The described system is highly adapted to be 1 used in tubes with directional electron beams for generating ultra-short Waves. It may also serve excellently in cathode-ray tubes for the reception of electrically transmitted pictures and more particularly in such tubes which serve for the. a

projection of the transmitted picture with an enlarged size, for which a great luminous intensity and hence a high current density is re- 1. An electronldischarge tube comprising a --'target adapted to receive electrons, a cathode having a concave surface with respect to said target'to emit electrons, means adjacent said target to develop-an electrostatic field having 1 lines of force extending in a direction substantially normal to elemental areas of said concave cathode surface, and a tubular ferro-magnetic bodysurrounding said cathode with its end near said cathode surface to develop a magnetic field having over said cathode surface lines of force substantially coincident with the lines of force of 'saidelectrostatic field. a 2'; An electron discharge device comprising a target; an oppositely disposed cathode having an emitting surface concave to said target, a tubular ferro-magnetic body surrounding said cathode positioned with its end adjacent the rim of said surface to develop a magnetic field the fringe lines of which are substantially normal .to elemental areas of saidcathode surface and means adjacent said cathode to develop an electrostatic field having lines of force coincident .cathode, magnetic means surrounding and coextensive with the fringe lines directly adjacent said cathode.

3. An electron discharge tube comprising a target, a concave cathode facing said target and aligned along the axis of said cathode therewith, a ferro-magnetic body having a tubular pole piece surrounding and adjacent the rim of said cathode to develop a magnetic field having a fringing field directed toward said axis immediately adjacent the concave surface of said cathode, and an apertured electrode in said fringing field and adjacent said cathode having a concave surface substantially conforming to that of said cathode to develop an electrostatic field having lines of force substantially coincident with thelines of force of said magnetic fringing 4. An electron discharge device comprising an indirectly heated cathode having a concave emitting surface positioned to direct an electron beam along'an axis extending through said said cathode and having one of its poles substantially flush with the rim of the concave emitting surface of said cathode todevelop a magnetic fringe field extending fora short distance in the direction ofsaid axis and intercepting said cathode surface and a mesh electrode substantially equidistant from the concave surface of said cathodeto direct electrons along paths intercepting said axis along the fringe field developed by said magnetic means.

5. A cathode ray'tube comprising an evacuated envelope, a concave cathode, a target optially flush with the rim edge of the concave portion of said cathode.

FREDERIK COETERIER; 

